Hi!
That is an abscess. Visit a dentist and he/she will take an X-ray.
If the tooth has encountered any trauma or decay then it has to be addressed accordingly.
Medication is must along with treatment.
Next Steps
Visit a Pediatric Dentist at the earliest.
Health Tips
It looks like but not a pimple. Don't break it by self. Treatment by a dentist is needed. Take car.